Liquid crystal display apparatus

ABSTRACT

A liquid crystal display apparatus having a pair of substrates and a liquid crystal layer interposed between the pair of substrates. One of the pair of substrates has an electrode structure including a plurality of pixel electrodes and a plurality of common electrodes, an organic polymer film is formed on at least one of the pixel electrodes and the common electrodes, and the liquid crystal layer is in direct contact with the organic polymer film.

CROSS REFERENCE TO RELATED APPLICATION

This is a continuation of U.S. application Ser. No. 09/004,411, filedJan. 8, 1998, the subject matter of which is incorporated by referenceherein.

BACKGROUND OF THE INVENTION

The present invention relates to a liquid crystal display apparatus,and, more particularly, the invention relates to a liquid crystaldisplay apparatus employing a lateral electric field method which isdriven by applying an electric field in a direction nearly parallel tothe substrate surface.

In prior art liquid crystal display apparatuses, the electrodes whichare used for driving a liquid crystal layer are transparent electrodesdisposed opposite to each other and respectively formed on twosubstrates. This structure comes from employing a display methodtypified by a twisted nematic display method in which the direction ofthe electric field applied to the liquid crystal is orientated in adirection nearly normal to the substrate surface. On the other hand, amethod of employing comb-shaped electrodes, in which the direction ofthe electric field applied to the liquid crystal is orientated in adirection nearly parallel to the substrate surface, is proposed, forexample, in Japanese Patent Publication No. 63-21907, U.S. Pat. No4,345,249, WO 91/10936, Japanese Patent Application Laid-Open No.6-22239-7 or Japanese Patent Application Laid-Open No. 6-160878. In thiscase, the electrodes do not need to be transparent, and so highlyconductive, opaque metal electrodes are employed. In regard to theabove-mentioned prior art display method in which the direction of theelectric field applied to the liquid crystal is orientated in adirection nearly parallel to the substrate surface (hereinafter,referred to as the “lateral electric field method”), a method ofreducing unevenness in the display, such as a domain on the electrodeedge existing from the initial stage of use, has been proposed inJapanese Patent Laid-Open No.7159786, but there is no descriptiontherein concerning the elimination of unevenness in the display, whichis seen as black spots produced during long-term use, or concerning thestructure necessary for improving the productivity of the apparatus.

SUMMARY OF THE INVENTION

The inventors of the present invention have newly found that when aliquid crystal display apparatus of the lateral electric field type isused continuously for a long time, unevenness in the display, seen asblack spots (hereinafter referred to as “black stains”), is produced. Asa result of studying the cause of the black stains, it has been revealedthat the black stains are produced by the following process.

(1) The liquid crystal is directly in contact with an electrode througha crack in a protective film.

(2) Electrochemical reaction occurs on the electrode due to a signalvoltage applied to the electrode to produce an ionic substance.

(3) The produced ionic substance exudes onto the liquid crystal layer toreduce a voltage holding ratio, and consequently the spot is seen asblack.

It can be seen from the above-mentioned considerations that the problemof black stain production can be solved by preventing any direct contactbetween the liquid crystal layer and the electrode.

An object of the present invention is to provide a liquid crystaldisplay apparatus in which black stains are not produced. Another objectof the present invention is to provide a method of manufacturing aliquid crystal display apparatus without producing black stains and withan increased productivity.

According to the present invention, by forming a thick protective filmor thin electrodes, cracks do not occur in the protective film, andconsequently the occurrence of black stains can be prevented. The cracksin the protective film are formed by cracks in a thin portion of thefilm due to a stress applied to the film. Particularly, the cracks areapt to occur in edge portions of the electrodes because thickness of theprotective film there is thin. The reason for this is as follows.

An SiN film commonly used for the protective film has a characteristicof film growth only in a direction normal to the substrate. Therefore,in the case of a common electrode cross-sectional structure, where theedge portion of the electrode is nearly 90 degrees or a reentrant etch,the edge portion of the electrode cannot be covered with the protectivefilm until the film thickness of the protective film becomes thickerthan the film thickness of the electrode. Accordingly, the filmthickness of the protective film at the edge portion of the electrode isdetermined by the difference between the film thickness of theprotective film and the film thickness of the electrode film.

Therefore, in order to form a sufficiently thick protective film at theedge portion of the electrode, to prevent occurrence of the cracks, itis necessary that the film thickness of the protective film issufficiently thicker than the film thickness of the electrode. Theinventors of the present invention have concentratively studied thedifference between the film thickness of the protective film and thefilm thickness of the electrode film necessary for eliminating blackstain faults, and they have found as a result that the thickness of theprotective film should be thicker than the film thickness of theelectrode under the protective film by more than 0.4 μm.

Further, according to the present invention, the black stain faults canbe eliminated by forming the protective film using an organic polymerfilm, instead of using an inorganic film formed through a film formingmethod under vacuum, such as the CVD method. The organic polymer filmcan be formed through the common wet method using a solution.

The wet film-forming method using a solution is composed of the threeprocesses of (1) forming the solution film over the whole surface, (2)drying the solvent and flowing in the lateral direction of the solution,and (3) forming the film by drying the solvent. Since the film is formedover the whole surface of the substrate by the process (1) and thesolution flows from the upper portion of the electrode to the lowerportion to make the film thickness uniform by the solution flow in theprocess (2), a film which is relatively thick can be formed also in theedge portion of the electrode, as compared to the case of an inorganicfilm. The organic polymer film is so soft that cracks hardly occur,compared to an inorganic film, such as a SiN film.

For the above-mentioned reason, it is possible to reduce the black staindefect by using an organic polymer film for the protective film.

The materials capable of being used for the organic polymer film arevarious kinds of organic polymer, such as polyimide, acrylic polymer,epoxy polymer and benzocyclobutene polymer.

Further, according to the present invention, it is possible to suppressthe occurrence of black stain faults by employing an electrode having atapered etch. Since film growth takes place, even on an electrode havinga tapered etch, from the initial stage, a protective film having asufficient thickness also can be formed in the edge portion. Therefore,the occurrence of cracks can be suppressed to reduce the black stainfaults. An electrode with a tapered etch can be formed by using anappropriate etching gas or an appropriate etching solution. For example,for a chromium electrode, an electrode with a tapered etch can beobtained by using an etching solution of cerium-sulfate-ammonium or thelike. For an electrode made of MoTa, an electrode with a tapered etchcan be obtained by using a mixed gas of CF₄ and O₂.

Furthermore, according to the present invention, it is possible tosuppress the occurrence of black stain faults and reduce the filmthickness of the protective film by employing a sufficiently thickalignment film without pin holes. That is, it is possible to eliminateblack stain faults without decreasing the productivity.

Since the alignment film is formed as a uniform film without pin holeswhen the film thickness of the alignment film becomes a certain value,the alignment film can not only orientate the liquid crystal, but canserve as a protective film. Therefore, by forming the alignment film sothat it is thick, it is possible to suppress the occurrence of blackstains even if the protective film is thin. The alignment film having asufficient film thickness can be obtained by a method of increasing theconcentration of the solution without increasing the film-forming time.Therefore, increasing the thickness of the alignment film and reducingthe thickness of the protective film shortens the film-forming time ofthe protective film. That is, there is an effect of improving theproductivity.

The inventors of the present invention have concentratively studied thefilm thickness of the alignment film necessary for eliminating pin holesas well as for eliminating black stain faults, and they have found as aresult that the alignment film should be thicker than 0.1 μm.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, features and advantages of the presentinvention will be understood more clearly from the following detaileddescription when taken with reference to the accompanying drawings,wherein:

FIG. 1 is a cross-sectional view showing a first embodiment of anelectrode substrate in accordance with the present invention;

FIG. 2 is a plane view showing the structure of various kinds ofelectrodes in a unit pixel of FIG. 1;

FIG. 3 is a graph showing the relationship between the thickness of theprotective film and the number of black stains in the first embodimentin accordance with the present invention;

FIG. 4 is a cross-sectional view showing a second embodiment of anelectrode substrate in accordance with the present invention;

FIG. 5 is a cross-sectional view showing a third embodiment of anelectrode substrate in accordance with the present invention;

FIG. 6 is a cross-sectional view showing a fourth embodiment of anelectrode substrate in accordance with the present invention;

FIG. 7 is a graph showing the relationship between the thickness of theprotective film and the number of black stains in the fourth embodimentin accordance with the present invention;

FIG. 8 is a schematic cross-sectional view of a liquid crystal displayapparatus to which the present invention is applied;

FIG. 9 is a schematic circuit diagram showing the construction of adriving system of a liquid crystal display apparatus in accordance withthe present invention; and

FIG. 10 is a diagram showing the relationship among a rubbing direction,an applied electric field direction and a direction of the polarizingplate transmission axis in a liquid crystal display apparatus accordingto the present invention; and

FIG. 11 is a cross-sectional view of a liquid crystal display apparatuscorresponding to FIG. 8 and showing a protective film which serves as analignment film in contact with at least one pixel electrode.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[First Embodiment]

FIG. 1 is a cross-sectional view showing a first embodiment of anelectrode substrate in accordance with the present invention. A commonelectrode 2 and a scanning electrode 3 made of aluminum are formed on aglass substrate 1, and further, the surface is covered with an aluminafilm 4. A gate insulator film 5 made of SiN is further formed on theelectrodes, and on the gate insulator film, an amorphous Si (a-Si) film6, an n-type a-Si film 7 and a thin film transistor (TFT) composed of asignal electrode 8 and a pixel electrode 9 made of Al/Cr are formed. Onthe upper layer, a protective film 10 made of SiN is formed, and on thefurther upper layer, an alignment film 11 is formed.

FIG. 2 is a plane view showing the structure of various kinds ofelectrodes in a unit pixel in the first embodiment of the presentinvention. The common electrode 2 is arranged in parallel to thescanning electrode 3. The pixel is divided into four portions by thecommon electrode 2 and the pixel electrode 9, which are arranged inparallel to the image signal electrode 8. The pixel electrode 9partially overlaps with the common electrode 2 to form a holdingcapacitance.

A characteristic of the present embodiment is that the thickness of theprotective film 10 is greater than the thickness of the image signalelectrode 8 and the thickness of the pixel electrode 9 by more than 0.4μm. When the thickness of the protective film is sufficient, crackshardly occur in the protective film and accordingly black stain defectsdo not take place. A result of confirming the occurrence of black stainsin liquid crystal display apparatuses having protective films ofdifferent film thickness shows that when the thickness of the protectivefilm is greater than the thickness of the signal electrode by more than0.4 μm, black stain defects do not take place, even if the filmthickness of the image signal electrode is varied from 0.2 μm to 0.6 μm,as shown in FIG. 3.

As described previously, the reason for this can be surmised as follows.When the thickness of the protective film is greater than the thicknessof the signal electrode by more than 0.4 μm, the thickness of theprotective film at the edge portion of the electrode becomessufficiently thick. It can be considered that cracks do not occur in theprotective film and consequently the occurrence of black stains can beprevented.

It can be understood from the above-mentioned explanation that theelectrode materials of the scanning electrode, the common electrode, thepixel electrode and the signal electrode are not limited to theaforementioned materials, and any metallic material may be employed ifit has a low electric resistance. For example, a metal, such aschromium, aluminum, copper, niobium or the like, or an alloy such asCrMo, AlTiTa or the like, may be employed.

[Second Embodiment]

FIG. 4 is a cross-sectional view showing a second embodiment of anelectrode substrate in accordance with the present invention. Acharacteristic of the present embodiment is that an organic polymer isemployed for the protective film. The organic polymer film can be formedthrough the common wet method using a solution. At that time, the filmthickness is made uniform by means of the solution flow in thefilm-forming process, that is, there is a so-called step reducingeffect. Therefore, a film having a relatively large thickness can beformed also in the edge portion of the electrode compared to the case ofthe film-forming method under a vacuum environment, such as the CVDmethod. Accordingly, cracks hardly occur. Further, an organic polymerfilm is so soft compared to an inorganic film, such as a SiN film, thatcracks hardly occur from this point.

It can be understood from the above-mentioned explanation that use of anorganic polymer material is not a limitation of the invention, but it ispossible to employ any material for forming the protective film througha wet method using a solution. For example, a benzocyclobutene polymer,epoxy polymer or polyimide polymer may be employed. Particularly, when apolyimide polymer is employed, the process time can be shortened, andaccordingly the productivity can be improved, because the protectivefilm can also serve as the alignment film. The protective film servingas an alignment film is illustrated in FIG. 11 which corresponds to theillustration in FIG. 8 and such film 24 is in direct contact with atleast one of the pixel electrodes 9 and the liquid crystal layer 12.

[Third Embodiment]

FIG. 5 is a cross-sectional view showing a third embodiment of anelectrode substrate in accordance with the present invention. Acharacteristic of the present embodiment is that the cross section ofthe signal electrode and the pixel electrode has a tapered etch.Therefore, the protective film is formed even in the edge portion of theelectrode from the initial stage, and so the film at the edge portionbecomes thick. Therefore, the occurrence of cracks can be suppressed toreduce the black stain faults.

An electrode with a tapered etch can be formed by using an appropriateetching gas or an appropriate etching solution. For example, for achromium electrode, the normal taper structure can be obtained by usingan etching solution of cerium-sulfate-ammonium or the like. For anelectrode made of MoTa, an electrode with a tapered etch can be obtainedby using a mixed gas of CF₄ and O₂.

In the present embodiment, the electrode materials of the scanningelectrode, the common electrode, the pixel electrode and the imagesignal electrode are not limited to the aforementioned materials either,since any metallic material may be employed if it has a low electricresistance. For example, a metal, such as chromium, aluminum, copper,niobium or the like, or an alloy such as CrMo, AlTiTa or the like, maybe employed.

[Fourth Embodiment]

FIG. 6 is a cross-sectional view showing a fourth embodiment of anelectrode substrate in accordance with the present invention. Acharacteristic of the present embodiment is that the protective film isthin and the alignment film is thick compared to that of the firstembodiment. Since the thick alignment film also serves as a protectivefilm, it is possible to suppress the occurrence of black stains even ifthe protective film is thin. If the thickness of the protective film canbe reduced, the total process time can be shortened and accordingly theproductivity can be improved.

As a result of studying different thicknesses of the alignment film, ithas been found that, if the film thickness of the alignment film isgreater than 0.1 μm, it is possible for the film thickness of theprotective film to be reduced without the occurrence of black staindefects (FIG. 7). It is assumed that the reason for this is that,although the alignment film having a film thickness of nearly 0.05 μm asin a common liquid crystal display apparatus is a non-uniform film withpin holes, the alignment film having a film thickness above 0.1 μmbecomes a uniform film without pin holes.

The present invention is not limited to the material of the alignmentfilm used in the present embodiment is not particularly limited, it ispossible to use 2,2-bis[4-(p-aminophenoxy) phenyl-propane as a diamine,polyimide using pyromellitic acid dihydride as an acid anhydride,paraphenylenediamine or diamino-diphenylmethane as an amine component, apolyimide using fatty tetracarboxylic acid dianhydride or a pyromelliticacid dianhydride as an acid anhydride component.

FIG. 8 is a schematic cross-sectional view of a liquid crystal displayapparatus to which the present invention is applied. On an oppositeglass substrate 13 in the upper portion, taking the liquid crystal layer12 as a base, there are formed a black matrix 14, a color filter 15, acolor filter protective film 16 and an alignment film 11.

On a glass substrate 1 in the lower portion, taking the liquid crystallayer 12 as a base, there are formed a common electrode 2 and a scanningelectrode 3, and the surface is covered with an alumina film 4. A gateinsulator film 5 made of SiN is further formed on the electrodes, and onthe gate insulator film, an amorphous Si (a-Si) film 6, an n-type a-Sifilm 7 and a thin film transistor (TFT) composed of a signal electrode 8and a pixel electrode 9 made of Al/Cr are formed, on the upper layer, aprotective film 10 made of SiN is formed, and on the further upperlayer, an alignment film 11 is formed. For the structure of thesubstrate having these electrodes, the structure shown in FIG. 1 andFIG. 2 according to the first embodiment is depicted by way of example.

As shown in FIG. 9, a drive LSI is connected to the liquid crystaldisplay apparatus, a vertical scanning circuit 17, a signal drivingcircuit 18 and a common electrode driving circuit 19 are connected ontothe TFT substrate, and a scanning voltage, a signal voltage and a timingsignal are supplied from a power source circuit and a controller 20 toperform active matrix drive.

FIG. 10 is a view showing the relationship among the rubbing direction,the applied electric field direction and the direction of the polarizingplate transmission axis in a liquid crystal display apparatus of thepresent embodiment. The rubbing directions 21 on the upper and the lowersubstrates are nearly parallel to each other, and the angles with theapplied electric field directions 22 are set to 75 degrees(φLC1=φLC2=75°). The liquid crystal display apparatus is interposedbetween two polarizing plates, and one of the polarized platetransmission axes 23 is set nearly parallel to the rubbing direction,that is, (φP1=75°, and the other is set so as to intersect with the oneof the polarized plate rubbing directions at a right angle, that is,φP2=−15°. By doing so, it is possible to obtain a normally closedcharacteristic.

By the above-mentioned structure, it is possible to provide a liquidcrystal display apparatus having a good display quality without blackstain defects.

By the above-mentioned structure, it is possible to provide a liquidcrystal display apparatus of the lateral electric field type withoutdisplay unevenness produced by black spots.

What is claimed is:
 1. A liquid crystal display apparatus comprising: apair of substrates; and a liquid crystal layer interposed between saidpair of substrates; wherein one of said pair of substrates has anelectrode structure including a plurality of pixel electrodes and aplurality of common electrodes, an organic polymer film is formed on atleast one of said plurality of pixel electrodes and said plurality ofcommon electrodes, and said liquid crystal layer is in direct contactwith said organic polymer film, and wherein said organic polymer filmformed on said at least one of said plurality of pixel electrodes andsaid plurality of common electrodes is in direct contact with said atleast one of said plurality of pixel electrodes and plurality of commonelectrodes and said organic polymer film serves both as a protectivefilm for said at least one of said plurality of pixel electrode and saidplurality of common electrode and an alignment film for said liquidcrystal layer.
 2. A liquid crystal display apparatus comprising: a pairof substrates; and a liquid crystal layer interposed between said pairof substrates; wherein one of said pair of substrates has an electrodestructure including a plurality of pixel electrodes and a plurality ofcommon electrodes, a protective film is formed on at least one of saidplurality of pixel electrodes and said plurality of common electrodes,said protective film is an organic polymer film, and said liquid crystallayer is in direct contact with said protective film, and wherein saidprotective film being said organic polymer film formed on at least oneof said plurality of pixel electrodes and said plurality of commonelectrodes is in direct contact with said at least one of said pluralityof pixel electrodes and said plurality of common electrodes and furtherserves as an alignment film for said liquid crystal layer.